Referring to FIG. 1, a beam pump or pumpjack, designated generally as 10, is the mechanical drive that converts rotary motion of a motor or prime mover 12 to reciprocating motion for reciprocating a downhole piston pump 14 in an oil well. Sucker rods 16 join the surface components of beam pump 10 and the downhole piston pump 14. Downhole pump 14 typically includes a pump barrel 17, which contains a plunger 21 carrying a travelling valve 18, and a standing valve 20. Beam pump 10 is used to mechanically lift liquid out of a well if there is insufficient bottom hole pressure for liquid to flow all the way to the surface. Beam pump 10 may also be used to increase the current production from a low producing well. Beam pumps, such as beam pump 10 of FIG. 1, are commonly used for low producing onshore wells and are common in oil-rich areas. About 60% of all artificially lifted wells in North America are beam or sucker rod pump systems and the figure is closer to 70% worldwide.
Depending on the size of beam pump 10, production may be from 15 to 30 liters of liquid per stroke, depending on design parameters. Often, the produced fluid is a mixture of crude oil and water with the possibility of some gas. Pump unit size, the size of sucker rods 16, and the horsepower capability of prime mover 12 are selected to accommodate the depth and weight of the oil to be removed. Deeper extraction requires greater power to move the increased weight of the discharge column. The diameter of downhole pump 14 and stroke length of the surface unit of beam pump 10, along with the pumping speed, i.e., strokes per minute (SPM), determine the producing rate of liquids. Liquids are routed up tubing string 24.
Beam pump 10 converts the rotary motion of a pump motor 12 to a vertical reciprocating motion to drive sucker rods 16, which are connected to the piston pump or downhole pump 14. The vertical reciprocating motion of beam pump 10 produces the characteristic nodding motion of the pump, which may be referred to as a walking beam.
A surface dynamometer card is the plot of measured or predicted surface loads on rods 16 of the pump shaft, i.e., sucker rods, at various positions throughout a complete stroke of beam pump 10. Surface loads may be measured via a load cell, e.g., located under a rod clamp resting on a carrier bar. Alternatively, a predicted surface load may be obtained from a predictive wave equation computer program, as is known in the art. For purposes of this application, a load cell, or other load measuring device, as well as computer or software that calculates surface load, shall be referred to as a load calculator. A surface dynamometer card reflects forces at the surface but can also be used to calculate and to plot forces in rods 16 above downhole pump 14 or anywhere in the string of sucker rods 16 as a function of position at the bottom of rod string 16 or anywhere in rod string 16. The loads on the surface card or loads in the rods 16 at the surface are a result of the fluid load and also are a result of the weight of rods 16 in fluid and dynamic forces. The load is typically displayed in pounds of force (Y scale) and the position (X scale) of a rod is typically displayed in inches. Dynamometer cards are displayed by predictive and diagnostic software for the purposes of design and diagnosis of sucker rod pumping systems to show stroke length, maximum/minimum loads for a cycle and other parameters.
Some diagnostics may be conducted by an analysis of surface dynamometer card shapes, since certain downhole problems are typically associated with particular surface dynamometer card shapes. In shallow to medium depth wells, such interpretation of the surface dynamometer card may be reasonably effective in diagnosing pump performance. In deeper wells, however, the complex nature of the lift system means that diagnosing pump performance from surface dynamometer cards can be more problematic due to the dynamics of the long string of sucker rods.
A downhole dynamometer card, designated generally 30 (FIG. 2), is a plot of calculated loads at various positions of pump stroke and represents the fluid load that pump 14 applies to the bottom of the rod string 16. Downhole dynamometer card 30 has four indices, i.e., A, B, C, and D, representing opening and closing events of standing valve 20, i.e., indices B and C, and opening and closing events of travelling valve 18, i.e., indices A and D. A schematic of pump 12 is shown adjacent to each labeled corners A-D of card 10 wherein the status of pump 14 at each of points A-D is shown. The maximum plunger travel (MPT) is the maximum length of the movement of plunger 21 with respect to barrel 17 of pump 14 during one complete stroke. Most of the load, presented on the Y-axis of downhole dynamometer card 30, is a force caused by differential pressure acting on plunger 21 of pump 14 or the fluid load at pump 14. The differential pressure acts across traveling valve 18 on the upstroke and is transferred to standing valve 20 on the down stroke. The differential pressure is the difference between the pressure due to fluids within tubing 24 and the pressure in the wellbore. The magnitude of the fluid load is equal to the pump discharge pressure minus the pump intake pressure multiplied by the plunger area. Loads are shown on a downhole dynamometer card 30 on the Y scale, i.e., load in rod 16 above pump 14, and position of rods 16 above the pump 14 (X scale) will be transferred to a surface dynamometer card along with the weight of rods 16 in fluid and dynamic loads. A typical surface dynamometer card 40 is shown in FIG. 3.
Still referring to FIG. 2, the successive steps in the downhole pump operation include the following: At the start of the upstroke (point A), traveling valve 18 and standing valve 20 are both closed.
Still referring to FIG. 2, from points B to C, rods 16 carry the fluid load when traveling valve 18 is closed. From points D to A, tubing 24 carries the fluid load, when standing valve 20 is closed. The effective plunger travel (EPT) is the length of travel of plunger 21 when the full fluid load is acting on standing valve 20. In FIG. 2, the effective travel of plunger 21 is from B to C and is usually a smaller length than the surface stroke length due to stretch of rods 16.
Referring now to FIG. 3, a typical surface dynamometer card is shown. A surface dynamometer card is a plot of measured loads on rods 16 at various positions throughout a complete stroke. The load may be displayed in pounds of force and the position may be displayed in inches. With reference to surface dynamometer card 40, from point A to point B, the fluid load is fully carried by tubing 24 prior to point A and is gradually transferred rods 16 at point B. The load transfers as rods 16 are loaded and exhibit stretch to pick up the fluid load. If tubing 24 is anchored, plunger 21 and travelling valve 18 do not move relative to tubing 24. Pressure in pump 14 decreases and any free gas in the clearance space between valves 18 and 20 expands from the static tubing pressure (Pt) to the pump intake pressure (Pint).
Standing valve 20 begins to open at A, allowing fluid to enter pump 14 when the pressure in pump 14 drops below the intake pressure (Pint).
Still referring to FIG. 3, with reference to surface dynamometer card 40, from point B to C, the fluid load is carried by rods 16 as well fluids are drawn into pump 14. At C, standing valve 20 closes as plunger 21 starts down, and traveling valve 18 remains closed until the pressure inside pump 14 is slightly greater than the pump discharge pressure (Pd). From C to D, gas in pump 14 (if present) is compressed as plunger 21 moves down to increase pressure on the fluid from the intake pressure (Pint) to the static pressure in tubing 24. However, plunger 21 does not move if pump barrel 17 is full of an incompressible fluid. As fluid in pump barrel 17 is compressed, then the fluid load is gradually transferred from rods 16 to the tubing 24.
At D, the pump discharge pressure (Pd) equals the static tubing pressure (Pt), and traveling valve 18 opens. From D to A, fluid in pump 14 is displaced through traveling valve 18 into tubing 24 and the fluid load is held by tubing 24.